Types of Cable Faults:
An unsatisfactory feature and performance of cable used in the electrical installation are termed cable faults. There are various types of cable faults such as earth faults, short circuit faults, and open circuit faults. Here we will be discussing the types of cable faults, their causes, and tests to be performed to locate the cable faults.
Following are the types of faults which are most likely to occur in the cables.
1. Ground or Earth Faults:
The insulation breakdown of the cable results in in-ground or earth faults. Due to this current starts to flow from the core of the cable to the earth or the cable sheath.
2. Cross or Short-Circuit Faults
The insulation breakdown between the two cables or in a multi-core system cable the insulation breakdown between the two cores results in a cross or short-circuit fault. Due to this current starts to flow from one cable to another or from one core to the other.
3. Open-Circuit Fault
Open-circuit cable faults result due to the breakage of the cable and no current flow through the cable.
Determination of Nature of Fault
First, we have to determine the nature of the fault, to locate the fault.
Then to determine the nature of the fault we have to measure the insulation resistance of each core to the ground and between the cores of the cables. To measure the insulation resistance we require an instrument known as Meggar.
The indication of ground fault gives a low value of insulation resistance between any core and the earth.
The indication of short-circuit fault gives the low value of insulation resistance between two cores.
In the case of an open-circuit fault. To determine the nature of the fault, the far end of the cable is earthed (in the case of single cables). For multi-core cables, we need to interconnect the far end of the cables. Then, an ammeter is inserted in series with the cable and a low voltage supply is connected across the cable and earth or cables. The zero or no deflection in the ammeter indicates an open-circuit fault.
Causes of Cable Faults
Cable faults are mostly due to the dampness in the paper insulation due to penetrable or damaged lead sheath.
The lead sheath is damaged due to chemical action on the lead when the cables are buried in the earth. Also, the crystallization of lead through vibration and insufficient protection provided to the cable resulted in mechanical damage.
Location of Cable Faults for Ground Fault of Single Cable
For a single cable, to determine to locate the ground cable faults we use Blavier’s test.
This test is performed with a voltmeter or ammeter or bridge network.
Cable Faults Test
We need to perform loop tests for locating ground cable faults or short circuit faults in underground cables.
Unless the resistance of the fault is very high, the resistance of the fault does not affect the results of the loop test. Murray loop test and Varley loop test are two common loop tests method. These tests work on the principle of the Wheatstone bridge.
Fall of the potential test method is performed in the case when a second cable is free from the faults and runs along the faulty cable. This method of cable faults location is not as accurate as that of the Murray loop test.
In case of locating short-circuit faults on the cable, we have to measure the capacitance of the faulty cable from both ends by the use of a ballistic galvanometer or by an ac bridge. The essential requirement for this test is that the core of the cable to be tested should have good insulation resistance.